(1) Field of the Invention
The present invention relates to an optical pickup device which detects a reflection beam from an optical disk and generates a read-data signal based on the reflection beam.
(2) Description of the Related Art
A conventional optical pickup device for reading out data from an optical disk is known. In the conventional optical pickup device, a laser beam is emitted by a semiconductor laser to the optical disk. The emission beam from the semiconductor laser is converted into a converging beam by an objective lens, the converging beam passing through a transparent layer of the optical disk and forming a light spot on a recording surface of the optical disk. In the optical pickup device, a photodetecting unit generates a signal indicative of read-out data by detecting a reflection beam from the optical disk.
In recent years, optical disks such as compact disks (CD) are widely used to store information and reproduce the information. In the near future, two different types of the optical disks will come to be popular: a standard recording-density optical disk and a high recording-density optical disk. One of the basic requirements for these optical disks is that the standard recording-density type has a transparent layer which is about 1.2 mm thick, and the high recording-density type has a transparent layer which is about 0.6 mm thick.
It is desirable to provide an optical pickup device which can access the standard recording-density optical disk and the high recording-density optical disk in common by using the same pickup device, in order to enable data to be read out from each of the two optical disks.
Generally, in the optical pickup device, an emission beam from the semiconductor laser is converted into a converging beam by the objective lens, the converging beam passing through the transparent layer of the optical disk and forming a light spot on the recording surface of the optical disk. When the standard recording-density optical disk is accessed, the diameter of the light spot has to be about 1.5 .mu.m. When the high recording-density optical disk is accessed, the diameter of the light spot has to be about 0.95 .mu.m.
Since the diameter of the light spot on the recording surface of each of the two optical disks requires a high accuracy, it is necessary that optical characteristics (e.g., spherical aberration) of the objective lens be suitably preset depending on the thickness of the transparent layer of each of the two optical disks.
In order to provide an optical pickup device capable of suitably accessing both the two optical disks, it is conceivable that the optical characteristics of the objective lens are preset depending on the smaller thickness of the transparent layer of the high recording-density optical disk.
When the high recording-density optical disk is accessed by using the above objective lens, the converging beam from the objective lens can correctly pass through the transparent layer (the thickness: about 0.6 mm) of the optical disk and form a light spot which is suited to the requirement for the high recording-density type.
However, when the standard recording-density optical disk is accessed, the converging beam from the above objective lens cannot suitably pass through the transparent layer (the thickness: about 1.2 mm) of the optical disk and form a light spot which is suited to the standard recording-density optical disk. In particular, the converging beam from the above objective lens does not converge on the recording surface of the standard recording-density optical disk. Since the spherical aberration is poor due to the different thickness of the transparent layer of the optical disk, a suitably focused light on the recording surface of the optical disk is not formed.
In a conventional method for eliminating the problem of the above-mentioned optical pickup device, an aperture unit with an opening is placed on an optical path of the emission beam from the objective lens to the optical disk when the standard recording-density optical disk is accessed. The aperture unit has not to be placed when the high recording-density optical disk is accessed. When the standard recording-density optical disk is accessed, the aperture unit is placed on the optical path of the emission beam. This allows a desired portion of the converging beam from the objective lens to be directed to the optical disk, but inhibits an undesired portion of the converging beam from entering the optical disk, so as to avoid the above-described problem.
However, when an optical pickup device in which the above method is incorporated for practical use is taken into consideration, the position at which the aperture unit is placed on the optical path of the emission beam requires a very high accuracy. In order to meet this requirement, it is necessary that the above optical pickup device include an aperture moving mechanism which very accurately moves the aperture unit onto the optical path of the emission beam. When the above optical pickup device is produced, an additional process for producing the aperture moving mechanism must be performed. Further, the aperture moving mechanism and the aperture unit must require a very high accuracy of the assembly. Therefore, the above-described optical pickup device is not useful to reduce the cost of the optical pickup device or allow efficient production processes thereof.